Handsets nearby feed location data to the devices as if they were cell towers

Law enforcement agencies in California are using devices that mimic cellular base stations to track mobile users, public records have revealed, triggering charges that the practice may be unconstitutional.

Agencies in Los Angeles, San Diego, San Jose, San Francisco, Sacramento and other areas own or have funding to buy the so-called "stingray" devices, according to documents uncovered in an investigation by TV station Sacramento News 10. The American Civil Liberties Union of Northern California highlighted the findings in a blog post on Thursday, saying the use of stingrays may violate constitutional restraints on searches.

Stingrays, which have been reported in use in Florida, Colorado and other states, are small portable devices that appear to nearby phones as if they are real cellular base stations. When a stingray is nearby, phones will automatically connect to it as if it were the nearest cell tower. Law enforcement most commonly uses the devices to track the location of phones, though there are stingrays that can monitor calls, Staff Attorney Linda Lye wrote in the blog post.

The government's use of digital technology for widespread data-gathering on citizens has focused mostly on federal entities such as the National Security Agency. But local law enforcement has also come under attack for the use of technologies such as stingrays and small, unmanned "drone" aircraft.

Stingrays can track users as they move around with their phones, with a level of accuracy that allows for tracking even inside buildings, according to the ACLU. Because they take in data from every phone in the area, instead of limiting the surveillance to actual suspects, the devices are constitutionally suspect, Lye wrote.

"There is a real question as to whether stingrays can ever be used in a constitutional fashion. They are the electronic equivalent of dragnet 'general searches' prohibited by the Fourth Amendment," Lye wrote.

However, as with other types of surveillance, regulations and case law haven't kept up with technology, according to the ACLU. There are no laws governing law enforcement's use of stingrays, Lye wrote.

Most of the biggest local law-enforcement agencies in California are named in the report, including the Los Angeles Police Department and L.A. County Sheriff's Department, the San Diego Police Department, the San Francisco Police Department and the San Jose Police Department.

The federal government has funded stingrays for anti-terrorism efforts on the local level. For example, in 2013, San Jose received about US$250,000 for one or more stingrays from the Department of Homeland Security's Urban Areas Security Initiative. That program is designed to head off terrorism and other hazards. But in some cases the local agencies have brought the devices into ordinary law enforcement, such as the Oakland Police Department's use of stingrays in its Criminal Investigative Division, the ACLU said.

Because stingrays are typically funded through federal grants, they also bypass the local scrutiny of an agency's regular budget, according to the ACLU. If local citizens knew about such devices, they might block their use, Lye wrote.